1. Field of the Invention
The present invention relates to an electronic still camera for photographing a still picture by using a charge coupled solid-state image pickup device, and particularly relates to an electronic still camera for photographing a high-definition still picture obtained by reducing the influence of smear and dark current occurring in the charge coupled solid-state image pickup device and for producing a high-definition still picture without flicker.
2. Description of Background Art
Heretofore, conventional electronic still cameras have used a charge coupled solid-state image pickup device having vertical resolution of a level in accordance with a standard television system, such as an NTSC television system, with 525 scanning lines.
However, a conventional solid-state pickup device cannot attain the high vertical resolution suitable for a highquality television system which has a vertical resolution of about twice as much as that of an NTSC television system. Therefore, development of an electronic still camera using a solid-state image pickup device having a larger number of pixels has been desired.
The inventors in this application have investigated and developed an electronic still camera using an inter-line transfer charge coupled solid-state image pickup device having vertical direction pixels increased in number by about twice that of the conventional device, by which pixel signals from all the pixels are read by 4 field scanning operations.
The structure of the developed solid-state image pickup device is described with reference to FIG. 11. In the drawing, photodiodes A1, B1, A2 and B2 are arranged in the form of a matrix of 800,000 pixels having 1000 rows in the vertical direction X and 800 columns in the horizontal direction Y. Vertical charge transfer paths l.sub.1 to l.sub.m are formed adjacent to the respective columns of the photodiodes. A horizontal charge transfer path HCCD is formed in terminal portions of the respective vertical charge transfer paths l.sub.1 to l.sub.m. Pixel signals are read out successively in time series in synchronism with the scanning timing through an output amplifier AMP formed in a terminal portion of the horizontal charge transfer path HCCD.
The arrangement of the photodiodes is defined so that the photodiodes A1 arranged in the (4n-3)-th row (n being a natural number) correspond to the first field, the photodiodes B1 arranged in the (4n-2)-th row correspond to the second field, the photodiodes A2 arranged in the (4n-1)-th row correspond to the third field, and the photodiodes B2 arranged in the 4n-th row correspond to the fourth field. In short, the photodiodes are operated so as to read out all the pixel signals of one picture by carrying out four field scanning/reading operations in order.
A series of image pickup operations from an exposure operation to a pixel signal scanning and reading operation is described with reference to the timing chart of FIG. 12. The solid-state image pickup device performs a field scanning and reading operation in synchronism with the vertical synchronizing signal VD which expresses a field scanning period (1V). In short, in FIG. 12, a field shifting operation for every field is carried out in synchronism with the time of level inversion of the vertical synchronizing signal VD to an "H" level. (The period in which the level of each of the signals FS1, FS2, FS3 and FS4 is turned to an "H" level is a field shifting period for each field.) Thus, pixel signals corresponding to each field are read out in a 1V period before the level of the vertical synchronizing signal VD is turned to an "H" level again.
When, for example, the shutter release button is pushed at a point of time t.sub.1 in the second field scanning period (between t.sub.0 to t.sub.2) as shown in FIG. 12 to perform exposure for a predetermined shutter period (represented by "H" in the drawing), the operation of reading pixel signals is started from the field scanning and reading period just after the end of the exposure. In short, pixel signals q.sub.A2 generated in the photodiodes A2 corresponding to the third field are fieldshifted to the vertical charge transfer path side in a period in which the level of the signal FS3 is turned to an "H" level at the point of time t.sub.2. Then, the vertical charge transfer paths l.sub.1 to l.sub.m and the horizontal charge transfer path HCCD read out the pixel signals q.sub.A2 in time series in synchronism with a predetermined drive signal to carry out the third field scanning and reading operation in a period between t.sub.2 and t.sub.3.
Then, pixel signals q.sub.B2 generated in the photodiodes B2 corresponding to the fourth field are field-shifted to the vertical charge transfer path side in a period in which the level of the signal FS4 is turned to an "H" level at the point of time t.sub.3. Then, the vertical charge transfer paths l.sub.1 to l.sub.m and the horizontal charge transfer path HCCD read out the pixel signals in time series in synchronism with a predetermined drive signal q.sub.B2 to carry out the fourth field scanning and reading operation in a period between t.sub.3 and t.sub.4.
Then, pixel signals q.sub.A1 generated in the photodiodes A1 corresponding to the first field are field-shifted to the vertical charge transfer path side in a period in which the level of the signal FS1 is turned to an "H" level at the point of time thd 4. Then, the vertical charge transfer paths l.sub.1 to l.sub.m and the horizontal charge transfer path HCCD read out the pixel signals q.sub.A1 in time series in synchronism with a predetermined drive signal to carry out the first field scanning and reading operation in a period between t.sub.4 and t.sub.5.
Then, pixel signals qa generated in the photodiodes B1 corresponding to the second field are field-shifted to the vertical charge transfer path side in a period in which the level of the signal FS2 is turned to an "H" level at the point of time t.sub.5. Then, the vertical charge transfer paths l.sub.1 to l.sub.m and the horizontal charge transfer path HCCD read out the pixel signals in time series in synchronism with a predetermined drive signal q.sub.B2 to carry out the second field scanning and reading operation in a period between t.sub.5 and t.sub.6.
As described above, all the pixel signals of one picture are read out by carrying out four field scanning and reading operations just after the exposure.
However, in the case where pixel signals are scanned and read according to the timing as shown in FIG. 12, a large amount of smear contents are mixed in the pixel signals of the photodiodes corresponding to the field scanned and read just after the exposure. There arises a problem in that deterioration of picture quality is caused by field flicker produced at the time of reproducing a picture. For example, in the timing as shown in FIG. 12, a large amount of smear is mixed in the pixel signals q.sub.A2 corresponding to the third field scanned and read just after the exposure. The amount of smear mixed in the pixel signals corresponding to the fourth, first and second fields scanned and read after the third field scanning and reading period is not considerable. Accordingly, the luminance of the field picture corresponding to the third field becomes high and color irregularity occurs, thus resulting in the deterioration of picture quality.
A method for preventing the mixing of smear in the pixel signals of all fields is implemented by performing a scanning and reading operation of the vertical charge transfer paths l.sub.1 to l.sub.m and the horizontal charge transfer path HCCD after the exposure to thereby exhaust out unnecessary charges which cause smear and then, subsequently, carrying out an ordinary field scanning and reading operation. The above procedure is considered as a technique of preventing the mixing of smear in pixel signals.
In the following, the method is described with reference to the timing chart of FIG. 13. When exposure is carried out at a point of time t.sub.4, the transfer of pixel signals from photodiodes to the vertical charge transfer paths is prohibited by masking the field shifting operation (that is, the field shifting operation in the third field during the period T.sub.MS in the drawing) just after the exposure. In the period (between t.sub.5 and t.sub.6) for scanning and reading the original third field, a scanning and reading operation in a so-called empty reading state is performed in the vertical charge transfer paths and the horizontal charge transfer path, so that signals read out in the period (between t.sub.5 and t.sub.6) are aborted. As a result, smear contents in the vertical charge transfer paths and the horizontal charge transfer path are removed. Then, the fourth field scanning and reading operation is started at the point of time t to carry out the first, second and third field scanning and reading operations in order to thereby read out all the pixel signals for the four fields.
According to the scanning and reading method in the timing chart as shown in FIG. 13, the occurrence of smear can be reduced, however, the influence of dark current cannot be neglected. In short, dark current is a noise component which is always produced from surface portions of the semiconductor substrate and photodiodes. Accordingly, there is little influence of dark current on pixel signals (in FIG. 13, the pixel signals q.sub.B2 corresponding to the fourth field) first subjected to the field scanning and reading operation, because the period in which the pixel signals stay in the photodiodes is short. Dark current has influence on pixel signals (in FIG. 13, the pixel signals q.sub.A2 corresponding to the third field) subjected to the last field scanning and reading operation, because the period in which the pixel siqnals stay in photodiodes is long. In short, in the case of FIG. 13, dark current (represented by 4I) proportional to the 4V period between t to t is mixed in the pixel signals q.sub.B2 corresponding to the fourth field. Dark current (represented by 4I) proportional to the 4V period between t.sub.2 and t.sub.7 is mixed in the pixel signals q.sub.A1 corresponding to the first field. Dark current (represented by 4I) proportional to the 4V period between t.sub.3 and t.sub.6 is mixed in the pixel signals q.sub.B1 corresponding to the second field. Dark current (represented by 8I) proportional to the 8V period between t.sub.0 to t.sub.9 is mixed in the pixel signals q.sub.A2 corresponding to the third field, because the field shifting operation is prohibited at the point of time t.sub.5. As is obvious, the amount of dark current mixed in the pixel signals corresponding to the third field is larger. When a picture is reproduced on the basis of the pixel signals read out as described above, deterioration of picture quality is caused by luminance irregularity and color irregularity corresponding to the difference in the amount of mixed dark current between fields.